1. Field of the Invention
This invention concerns digital communications and, more specifically, communications wherein multiplexing techniques are used to concentrate several digital channels onto a single transmission channel.
2. Background of the Invention
Since manufacturing lines or, more generally, manufacturing communications channels, is an expensive operation, it is of interest to handle these lines to the best of their capacities. To this end, a number of techniques have been developed from amongst which are the so-called multiplexing techniques owing to which data coming from several channels are concentrated, on the transmission side, into a single common channel. The information conveyed by said common channel, must then be processed, on the reception side; more specifically, it must be demultiplexed, in order to send the received information to the addressees they are intended for. These addressees are either respectively connected to separate channels (point to point communications) or all connected to a same channel (multipoint communications). Hybrid systems are also provided wherein several channels are provided at the demultiplexer output, with some of these channels being connected in point to point configuration and others being connected in multipoint configuration. In any case, however, the configuration of the whole communications network, as a rule, must be perfectly defined and known of both the multiplexer and demultiplexer so that the pieces of information can be sent to their appropriate addressees, respectively, after their being conveyed onto said common channel. Besides, at any moment, some channels may be active and others be inactive. The configuration of the active channels should be adapted to be modified according to the needs so that the common channel can be handled for the best of the user's interest. That is why, though it is of interest to concentrate as many communications, i.e. channels, as possible, onto said common channel, the transmission conditions may require a reduction of the number of the simultaneously activated channels. In other words, the conditions, e.g. the atmospherics, may disturb the transmissions in a way such that only those communications which come from a number of channels lower than the total number of the channels connected to the system, will be concentrated onto said common channel. Therefore, some channels previously connected to the common channel will momentaneously be left inactive. It can easily be understood that when some channels are left inactive on the transmission side, (multiplexing side), the demultiplexer at the other end of the common channel must obviously be aware of that so that the received data can be sent to their addressees, respectively, i.e., so that the channel configuration on the Reception side be adapted to the channel configuration on the Transmission side.
Generally, a same channel is used for both the emission and reception operations. Consequently, the user, or the terminal connected thereto, fulfills both emission and reception functions. Therefore, the demand for modifying the configuration of the active channels could come from either of the ends of the common channel. In practice, however, the station, at one of the ends of the common channel, has a master function whereas the station at the other end, has a slave function. The decision to modify the active channel configuration as well as the choice of the best configuration at a given instant, will be made at the master station, and the slave will have to conform by modifying the configuration of the active channels which are connected thereto, accordingly. Of course, it results therefrom that the devices on the slave side must be aware of the decisions made by the master relative to the active channel configuration, and take the necessary steps for conforming to these decisions. A momentaneous interruption of the useful traffic may occur, which interruption must be reduced to a strict minimum.
Several solutions have already been proposed to solve at best the problems encountered for communicating to the slave the master's decision regarding the active channels configuration selected and for handling such a decision at the slave station. The simplest solution consists in placing an operator at each of the ends of the common communications channel, and providing each operator with a communications means for communicating with the other operator. The slave operator will modify the slave channels configuration according to the instructions received from the master operator. Though this solution is easy to implement, it is rather expensive for the user.
Another proposed solution consists in providing, within the common channel and outside the frequency range normally used therein for the transmission of the data coming from the above-mentioned channels, for a so-called lateral or secondary channel which will be solely reserved for transmitting from master to slave a special message transmitted at a relatively low rate (service speed) and which is indicative of the master's decision made for changing the configuration of active channels. The slave will modify the configuration of its channels in order to match the master's selected configuration, through successive approximations. Such a channel matching method is relatively slow. Besides, it has risks of disturbing the transmission system in that said lateral channel may interfere with the actual data channel.